Spinal cord stimulation (SCS) is a well-accepted clinical method for reducing pain in certain populations of patients. During SCS, the spinal cord, spinal nerve roots, or other nerve bundles are electrically stimulated using one or more neurostimulation leads implanted adjacent the spinal cord. While the pain-reducing effect of SCS is not well understood, it has been observed that the application of electrical energy to particular regions of the spinal cord induces paresthesia (i.e., a subjective sensation of numbness or tingling) that replaces the pain signals sensed by the patient in the afflicted body regions associated with the stimulated spinal regions. Thus, the paresthesia appears to mask the transmission of chronic pain sensations from the afflicted body regions to the brain.
In a typical procedure, one or more stimulation leads are introduced through the patient's back into the epidural space under fluoroscopy, such that the electrodes carried by the leads are arranged in a desired pattern and spacing to create an electrode array. The specific procedure used to implant the stimulation leads will ultimately depend on the type of stimulation leads used. Currently, there are two types of commercially available stimulation leads: a percutaneous lead and a surgical lead.
A percutaneous lead comprises a cylindrical body with ring electrodes, and can be introduced into contact with the affected spinal tissue through a Touhy-like needle, which passes through the skin, between the desired vertebrae, and into the epidural space above the dura layer. For unilateral pain, a percutaneous lead is placed on the corresponding lateral side of the spinal cord. For bilateral pain, a percutaneous lead is placed down the midline of the spinal cord, or two percutaneous leads are placed down the respective sides of the midline. A surgical lead has a paddle on which multiple electrodes are arranged typically in independent columns, and is introduced into contact with the affected spinal tissue using a surgical procedure, and specifically, a laminectomy, which involves removal of the laminar vertebral tissue to allow both access to the dura layer and positioning of the lead.
Stimulation energy may be delivered to the electrodes of the leads during and after the placement process in order to verify that the leads are stimulating the target neural tissue. Stimulation energy is also delivered to the electrodes at this time to formulate the most effective set of stimulus parameters, which include the electrodes that are sourcing (anodes) or returning (cathodes) the stimulation pulses at any given time, as well as the magnitude and duration of the stimulation pulses. During the foregoing procedure, an external trial neurostimulator may be used to convey the stimulation pulses to the lead(s), while the patient provides verbal feedback regarding the presence of paresthesia over the pain area. The stimulus parameter set will typically be one that provides stimulation energy to all of the target tissue that must be stimulated in order to provide the therapeutic benefit (e.g., pain relief, yet minimizes the volume of non-target tissue that is stimulated, which may correspond to unwanted or uncomfortable paresthesia. Thus, neurostimulation leads are typically implanted with the understanding that the stimulus parameter set will require fewer than all of the electrodes on the leads to achieve the desired paresthesia.
After the lead(s) are placed at the target area of the spinal cord, the lead(s) are anchored in place, and the proximal ends of the lead(s), or alternatively lead extensions, are passed through a tunnel leading to a subcutaneous pocket (typically made in the patient's abdominal area) where a neurostimulator is implanted. The lead(s) are connected to the neurostimulator, which is programmed with the stimulation parameter set(s) previously determined during the initial placement of the lead(s). The neurostimulator may be operated to test the effect of stimulation and, if necessary, adjust the programmed set(s) of stimulation parameters for optimal pain relief based on verbal feedback from the patient. Based on this feedback, the lead position(s) may also be adjusted and re-anchored if necessary. Any incisions are then closed to fully implant the system.
The efficacy of SCS is related to the ability to stimulate the spinal cord tissue corresponding to evoked paresthesia in the region of the body where the patient experiences pain. Thus, the working clinical paradigm is that achievement of an effective result from SCS depends on the neurostimulation lead or leads being placed in a location (both longitudinal and lateral) relative to the spinal tissue such that the electrical stimulation will induce paresthesia located in approximately the same place in the patient's body as the pain (i.e., the target of treatment). If a lead is not correctly positioned, it is possible that the patient will receive little or no benefit from an implanted SCS system. Thus, correct lead placement can mean the difference between effective and ineffective pain therapy.
Even after successful placement of the leads in the operating room (with corresponding test stimulation), the SCS system typically requires electrical fine-tuning post-operatively, and often it is difficult to target all pain areas, with some areas (e.g., the lower back) being particularly difficult to target. In particular, lead migration may relocate the paresthesia away from the pain site, resulting in the target neural tissue no longer being appropriately stimulated and the patient no longer realizing the full intended therapeutic benefit. With electrode programmability, the stimulation area can often be moved back to the effective pain site without having to reoperate on the patient in order to reposition the lead. For example, some SCS systems use changes in electrode polarity or incremental electrical current shifts in the cathodes and anodes to tune the location of paresthesia.
To produce the feeling of paresthesia without inducing discomfort or involuntary motor movements within the patient, it is often desirable to preferentially stimulate nerve fibers in the dorsal column (DC nerve fibers), which primarily include sensory nerve fibers, over nerve fibers in the dorsal roots (DR nerve fibers), which include both sensory nerve fibers and motor reflex nerve fibers. In order to stimulate the DC nerve fibers, while guarding against the stimulation of the DR nerve fibers, SCS systems may activate anodes that flank a single cathode in a medial-lateral electrical field, with the single cathode providing the stimulation energy for the DC fibers, while the flanking anodes guarding against the over-stimulation of the DR fibers, as illustrated in FIG. 1.
While change in the relative anode strengths will yield some tunability with a multiple source system, the electrical field is “tethered” to the single cathode, and so has limited flexibility in medial-lateral tuning. In fact, some hypotheses would suggest that “lower-back fibers” are off-midline, and thus a single cathode located over the center of the spinal cord may not be the optimum position for the cathode.
Also, the fixed spacing between the anodes and the cathode in a 3-column medial-lateral electrode arrangement is limiting, because the spacing would ideally be optimized to the distance from the electrodes to the spinal cord (due, e.g., to cerebral spinal fluid thickness (dCSF)), and that is a parameter with substantial variability between patients and at different vertebral levels within a patient. That is, in the case of a high dCSF, the spinal cord tissue will be relatively far away from the electrodes, and, therefore, it is desirable to increase the spacing between the anodes and cathode to lower the stimulation threshold by reducing the shunting of current, thus preventing excessive amplitudes. In the case of a low dCSF, the spinal cord tissue will be relatively close to the electrodes, and thus, current shunting (i.e., decay of field strength) is not as critical. In this case, it is desirable to increase the tunability of the stimulation by decreasing the spacing between the anodes and cathode. However, because the physical spacing between the anodes and cathode is fixed, and prior art SCS systems do not have the capability of electrically adjusting the spacing between the flanking anodes and the single cathode, variations in the dCSF cannot be suitably accounted for in prior art SCS systems. In addition, in a prior art medial-lateral arrangement, the electrodes are uniformly spaced and rostral-caudally aligned with each, which may not be the optimum arrangement.
There, thus, remains a need for an improved SCS system with improved targeting capability using a medial-lateral electrode arrangement.